"Punishment" in the Greek means the same as it means in English, to correct. Children are punished in order to learn, punishment is the means used to teach a lesson, break a spirit, prune to increase production. Punishment isn't an end, it's the methos used to bring a positive result.
Matt. 25:46 And these shall go away into everlasting punishment: but the righteous into life eternal.
Strong's Concordance1) correction, punishment, penalty 1) to lop or prune, as trees and wings 2) to curb, check, restrain 3) to chastise, correct, punishment 4) to cause to be punished